Update index.md (#2472)

Adding the Serverless unavailable tag, as per the [Elasticsearch table](https://www.elastic.co/docs/deploy-manage/deploy/elastic-cloud/differences-from-other-elasticsearch-offerings#elasticsearch) in the comparison doc.

Relates to [#1198](elastic/docs-content#1198)

Author: yetanothertw

docs/reference/index.md

@@ -4,6 +4,9 @@ mapped_pages:
   - https://www.elastic.co/guide/en/elasticsearch/hadoop/current/index.html
   - https://www.elastic.co/guide/en/elasticsearch/hadoop/current/doc-sections.html
   - https://www.elastic.co/guide/en/elasticsearch/hadoop/current/reference.html
+applies_to:
+  stack: ga
+  serverless: unavailable
 ---
 # {{esh-full}}
 
@@ -21,4 +24,4 @@ While the official name of the project is {{esh-full}} throughout the documentat
 This document assumes the reader already has a basic familiarity with {{es}} and Hadoop concepts. For more information, refer to [](./resources.md).
 % While every effort has been made to ensure that this documentation is comprehensive and without error, nevertheless some topics might  require more explanations and some typos might have crept in. If you do spot any mistakes or even more serious errors and you have a few cycles during lunch, please do bring the error to the attention of the elasticsearch-hadoop team by raising an issue or [contact us](http://www.elastic.co/community).
 % Thank you.
-::::
+::::